1. Field of the Invention
The present invention relates to a display device and a method of producing the same and, more particularly, to a display device and a method of producing the same in which reliability, durability, yield and the like are improved by preventing a strain and a crack from being generated in a substrate.
2. Description of the Background Art
Flat display devices represented by a liquid crystal display device (LCD) are used in various kinds of field because of characteristics such as light in weight, thin, and low power consumption. Among them, the liquid crystal display device is widely used in information devices represented by a personal computer. In addition, recently, the liquid crystal display device is widely used as a TV component and a conventional Braun tube is being replaced with it.
Moreover, a display device having a self-luminous light element in a display area, an electroluminescent (EL) display device, for example has been also used as a next-generation thin type display device. The EL display device has the characteristics such as wide viewing angle, high contrast, and high-speed response along with a moving image.
For example, many liquid crystal display devices are produced on a so-called mother glass substrate at once by multi-panel producing technology, in some cases. More specifically, a plurality of structures (cell structures) of the liquid crystal display devices are formed on the mother glass substrate larger than a glass substrate as a completed product, and the mother glass substrates are attached and bonded together. The bonded mother glass substrates are cut to provide an individual liquid crystal display device.
Recently, the substrate is required to be thin to reduce its weight in the display device. In addition, since a screen can be curved when the thin substrate is used, the substrate is required to be thin in view of the above point.
One of measures to satisfy the request of thinning is such that a thin glass substrate is used from the beginning of the production. However, a glass substrate having a thickness less than 0.5 mm is difficult to handle at the time of production because it is bent under its own weight or a crack is easily generated. Thus, a production yield deteriorates.
In addition, using a resin film and the like instead of the glass substrate is one idea. However, since the upper temperature limit of the resin film substrate is 200° C., there are constraints in view of a film forming temperature, etc.
In addition, a production method is disclosed in which a relatively thick glass substrate is used when the production process begins, and the glass substrate is etched away to be thinned during the production process (refer to Japanese Patent Application Laid-Open Nos. 2004-46115 and 2005-84228, for example). According to this production method, the above-described difficulty in handling can be reduced. In addition, the substrate can be thinner than 0.15 mm by etching, and can be curved so as to have a curvature radius of 200 mm. When an electric signal is applied while the substrate is curved, an image can be displayed on a curved screen.
The liquid crystal display of above Japanese Patent Application Laid-Open No. 2005-84228 includes a flexible substrate having at least one reinforcing part.
Japanese Patent Application Laid-Open No. 2007-94102 discloses a liquid crystal display device having flexible substrates which can be curved.
According to the liquid crystal display device produced by using the thin glass substrate from the beginning of the production, or thinning the glass substrate during the production, the problem is that the thin glass substrate is distorted. More specifically, according to the liquid crystal display device having the structure in which the thin glass substrates are bonded at their peripheries, when the screen is curved according to a design, or when the screen is curved for some reasons, the glass substrate is strained. Since the gap between the substrates, that is, the thickness of the liquid crystal layer at the distorted part differs from that of the other part, or since the opposed substrates are displaced, the image quality deteriorates.
In addition, according to the production method for producing many devices at once, the problem is that a fine crack (microscopic crack) is generated at the cut part, that is, the cut end part of the substrate. The microscopic crack could be a large crack when the substrate is curbed, for example. In addition, it is reported that the crack becomes large rapidly when water enters into the microscopic crack. Since the microscopic crack cannot be discovered by visual inspection or even by microscopic inspection in some cases, it could be a large crack while the liquid crystal display device is used. In addition, not only the microscopic crack generated when the substrate is cut, but also a crack generated at the end part of the substrate caused by various reasons during the production process could be a larger crack.
Here, the above problems could arise in a display device other than the liquid crystal display device, and in a substrate formed of a material other than a glass material.